For ISDN or other applications, a communication switching system is required to switch data carried in standard time slots, each corresponding to a channel, grouped in frames. This has been typically done in ISDN systems by concatenating standard 64 kb/s channels..
Unfortunately, in many prior art switching systems, different channels in the same bit stream experienced different delays during the switching operation. This resulted in data getting out of order and becoming corrupted. Such a system is unacceptable for ISDN applications.
Data enters a switch in specific channels, and is stored. It is then switched by reading the store, and leaves the switch in different channels. Since the different output channels are not in the same sequence as the incoming channels, different delays result from the assignment of time slots to the data from that of the incoming bit stream.
One of the ways of eliminating the out of order problem due to differing delays on different channels has been to provide a constant delay to the data. In the past, this involved buffering an entire frame of data and then switching this buffered data one frame later. This provided the constant delay by giving maximum delay to all channels.
Constant delay was a requirement for data packet switches. In such systems, the packets of data were extracted from the data stream and were then switched individually through a switching mechanism.
Many inputs share a common physical interface. A protocol is established so that each of the input and output devices can share the bandwidth of the bus, but each has exclusive use of the bus during its period. Newer mechanisms allow many packets to be switched at once, but still extract the packets before switching.